A frequent problem associated with color printers is misregistration or misalignment of one or more color planes. Alignment of the color planes is crucial in achieving a high quality image. The color planes are sequentially deposited onto a transfer medium such as an intermediate transfer belt that is used to transfer the color planes to medium such as a piece of paper. Alternately the medium itself may be transported and have the color planes sequentially deposited directly thereon.
Due to the fact that each individual color plane is transferred onto the belt or print medium at different locations along the travel path of the belt or print medium, the belt position within the travel path must be known or predicted with a high degree of precision. The position of the belt must be known to insure that the resulting image is of good quality.
There are many instances where belt positioning errors develop and cause a concomitant degradation in the resulting image. Drive roller runout, variations in the thickness of the belt, drive roller cylindrical imperfections, and variations in the belt tension are, in general, examples of factors that lead to belt positioning errors. In particular, the surface velocity of the belt is caused to run slower or faster depending upon whether: i) the belt is thin or thick as the belt passes over the drive roll; ii) the radius of the drive roller is longer or shorter as the belt passes over; and iii) the belt is tightly or loosely tensioned.
Others have tried to compensate for belt position errors by performing a calibration cycle within the printer at periodic intervals. The calibration cycle generates a test pattern from each color head to the transfer belt (typically toned line segments or symbols), detects the image position on the belt by way of a complex sensor, and corrects for belt speed or position based on the detected image. This manner of correcting for belt positioning to implement in the printer, wastes toner, and consumes time each occasion the calibration cycle is run. It would be desirable to have a method and apparatus that corrects for belt positioning errors which is inexpensive to implement in a printer, does not require user calibration, and does not add complexity to the printer.